Energy analyzer of parallel plane type

ABSTRACT

In an energy analyzer of parallel plane type used for determining the energy composition of a group of electrified particles which have passed a sample membrane and a part of which have been enervated or retarded, the analyzer comprising two planar electrodes disposed parallel with a space therebetween, a voltage being applied between said electrodes so as to deflect paths of the particles depending on the velocities thereof and to thereby separate the particles according to the magnitude of energy; an improvement for rendering the separation of particles virtually more complete by measuring secondary electrons emitted from one of the electrodes bombarded by particles having larger energy instead of measuring such particles per se.

2,769,911. 11/1956 Warmoltz ..250/4l.9

United States Patent 11 1 1111 3,710,102

Nagatani 1 Jan. 9, 1973 54 ENERGY ANALYZER 0F PARALLEL 3,609,353 9/1971 White ..2's0/41.9

PLANE TYPE 75 Inventor: Takashi Na atuni Katsuta,Ja an Primary Examiner lames Lawrence 1 g v p Assistant Examiner-B. C. Anderson 1 Asslgnw Hitachi Tokyo, Japan Attorney-Craig, Antonelli & 11 111 [22] Filed: Oct. 1, 1971 57 ABSTRACT [21] Appl. No.: 185,743 I 1 In an energy analyzer of parallel plane type used for determining the energy composition of a group of [30] Forms Apphcamm Priomy Data electrified particles which have passed a sample mem- Oct 5, 1970 Japan ..45/86640 r ne n a part of which have been enervated or re- M tarded, the analyzer comprising two planar electrodes [52] US. Cl 250/595 disposed parallel with a space therebetw een, a voltage 2 50/4l.9 ME, 250/49.5 E being applied between said electrodes so as to deflect I paths of the particles depending on the velocities thereof and to thereby separate the particles accord- Field of search-n-l5o/49-5 14 1 ing to the magnitude of energy; an improvement for '250/41-9 ME rendering the separation of particles virtually more complete by measuring secondary electrons emitted [56] Reierences cued from one of .the electrodes bombarded by particles 1 UNITED STATES PATENTS having larger energy instead of measuring such parti- 1 cles er se. 1 v 2,923,822 2/1960 Barnes et a1 ....250/4l.9 p

1 Claim, 4 Drawing Figures ENERGY ANALYZER OF PARALLEL PLANE TYPE BACKGROUND OF THE INVENTION mass spectrography of electrified particles such as electrons or ions, for example in studies of electron images transmitted through a sample membrane using an electronic microscope, it is generally required to determine the ratio in numbers of electrons which have undergone energy loss in the membrane to those which have not. Namely, assuming that electrified particles of N in number each having an energy of E are directed to a membrane to pass it, of which particles of n in number emerge from the membrane suffering no energy loss, that is, maintaining the initial energy of E while Nn particles undergo an energy loss of AE each, it is important to determine the ratio n /N-n in the mixed group of particles emerging from the membrane. For this purpose, the conventional energy analyzers of socalled parallel plane type have been used.

The conventional energy analyzer consists of two planar electrodes disposed parallel to each other, the first electrode being grounded while the second electrode is given a negative potential. The above-mentioned group of particles emerging from the sample membrane which include particles having the energy E, and those having the energy E, AE, are driven aslant into the space between the first and the second electrodes through an inlet provided in the first electrode. The electric field existing in the space between the electrodes acts on those electrified particles having different energies or velocities to deflect their paths along arcs. The resultant curvatures of the paths depend on the respective velocities of the particles, the path of particles with the larger energy (that is, E having smaller curvature than that of particles with the smaller energy (E, AB). The thus separated particles emerge from the space between electrodes through separate outlets provided in the first electrode to be detected by separate detectors placed outside of the first electrode.

Therefore, the ratio in numbers of particles having the energy E to those having the energy E AE can be determined by comparing output currents of the two detectors.

In the above-described conventional energy analyzer, however, the distance between the two outlets in the first electrode must be rendered too small to provide an adequate physical structure of the outlets, if the analyzer should be used for measurements in which the energy loss AB is expected to be very small. This have been a cause of various errors in the measurement.

SUMMARY OF THE INVENTION An object of this invention is to provide an energy analyzer of parallel plane type which can be easily constructed.

Another object of this invention is to provide such an analyzer with which the ratio in numbers of electrified particles enervated or retarded through a membrane to uninfluenced particles can be accurately determined.

In order to achieve the above objects, the parallel plane type energy analyzer of this invention used for analyzing the energy composition of a group of electrified particles, comprises a first planar electrode, a second planar electrode disposed parallel to said first planar electrode with a space therebetween, and means for applying a voltage between said first and second electrodes in such a manner that said second electrode is of the same polarity as that of said electrified parti cles in relation to said first electrode; said first electrode having a first aperture which allows said group of electrified particles to enter the space between said electrodes, a second aperture positioned so as to let pass secondary electrons emitted from said second electrode at the spot designed to be bombarded by a partial group of said particles having a larger energy and a third aperture positioned so as to let pass another partial group of said particles having a smaller energy which proceed along a curved path through said space between electrodes.

The energy analyzer of this invention will be described in detail hereunder with reference to the accompanying drawing and in comparison with the conventional energy analyzer.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram illustrating the conventional energy analyzer of parallel plane type.

FIG. 2 is a diagram useful for explaining the principle of parallel plane type energy analyzers.

FIG. 3 is a similar diagram for explaining the principle of the energy analyzer of this invention.

FIG. 4 is a sectioned schematic diagram of an embodiment of the energy analyzer of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 2 which illustrates the general principle of parallel plane type energy analyzer and which shows a section vertical to the two planar electrodes A and B disposed parallel and apart from each other by a distance d, the x axis (abscissa) is set for this explanation along the plane of the electrode A while the y axis (ordinate) is set to pass an aperture S in the electrode A and to be vertical to the planes of the electrodes. It is assumed that a voltage V is applied to the electrode A and a voltage V V to the electrode B.

If a grounded electron source is placed outside of the electrode A, electrons emitted from the source will be accelerated by the voltage V of the electrode A and attain a velocity v expressed by the following formula (1) when they reach the electrode A.

v,,= V (2e7m) V where e is electric charge carried by an electron; and

m is mass of an electron.

If an electron having such a velocity enters the electric field between the electrodes through the aperture S at an angle 0 to the y axis, path of the electron in said electric field traces a parabola expressed by the following equation.

Therefore, the value of x that gives maximum value of y is obtained as:

x,,,= (2d V /V sinO cost) (3) Accordingly, the maximum value of y is:

y d vV V cosO 4 Assuming that the electron deflected by the electric field strikes the electrode A at a point C, the distance x,- between the aperture S and the point C is determined as: ar 2 x It should be noted that if the initial velocity v, of the electron at the aperture S is rendered to become smaller than the value given by the above formula (1), then the values obtained by the above formulas (3), (4) and (5) become accordingly smaller. Namely, the distance x and the height y,,, are functions of initial velocity of the electron at the inlet S, if the voltage difference V,,, the distance d and the angle 0 are constant. Therefore, by providing apertures or electron outlets at different positions in the vicinity of the point C, it is possible to sort out electrons according to the initial velocity thereof. As an alternative measure, electrons having a particular initial velocity can be selected using only a single outlet by appropriately adjusting the voltage V applied to the electrode B. With the latter measure, however, concurrent measurements on electrons having different velocities or energies are impossible.

Referring to FIG. 1 which schematically shows the structure and operation of the conventional energy analyzer, a first and a second planar electrodes 1, 2 are I disposed parallel, the first electrode being grounded while the second electrode is given a negative potential. A group of electrified particles which include particles each having an energy E, and those having energy E, AE, are driveninto the space between the electrodes through an inlet 3 provided in the first electrode 1. Paths of the particles are bent by the electric field in the space, as described hereinbefore in connection with FIG. 2 curvatures of the particle paths differ depending on the initial velocity or energy of the respective particles, those having the larger energy E tracing a larger parabolic curve 4, while particles of the smaller energy E, AE, that is, enervated particles proceed along a smaller. parabolic curve 7. The particles of the larger energy emerge from the electrode assembly through an outlet 5 provided in the first electrode 1 and are detected by a detector 6. Similarly, the enervated particles come out through another outlet 8 to be detected by another detector 9.

Therefore, by comparing output currents of the detectors 6 and 9, the ratio in numbers of the enervated particles to the uninfluenced particles can be determined. With such a measure, however, the measurement becomes very difficult if the energy difference among particles is-small, because the distance between two outlets 5 and 8 becomes too small to fulfil necessary dimensional and structural requirements of the apertures. Thus, accurate measurements on particles having small energy difference are impossible with such a conventional analyzer.

For example, assume that electrons accelerated by an acceleration voltage of 50 kV are injected at an angle of 45 (angle 0 in FIG. 2) into the space between the electrodes which are mm apart from each other and are given a potential difference of 25 kV. In order to substantially intercept the electrons having a velocity corresponding to the acceleration voltage of 50 kV, a wall of 5 microns in thickness is required. Therefore, the space between two outlets. must not be less than 5 microns. A difference of 5 microns in the distance x (FIG. 2) corresponds to a difference of 2 eV in the initial energy of an electron. Thus, electrons having energy difference less than 2 eV cannot be separated by this analyzer. Attempt to increase the difference in the distance 1: for the small difference in electron energy will result in an extremely large size of analyzer which is too large to be of practical use.

The above difficulty has been overcome by this invention. The principle of this invention will be explained with reference to FIG. 3. It will be seen from the hereinbefore-shown formula (4) that the distance y,,,, that is the distance between the electrode A and the point D on the electron path which is remotest from the electrode A, is increased by decreasing the voltage difference V,,,, with the acceleration voltage V and the angle 6 unchanged. It will be clear from the formula (4) that the point D comes in contact with the electrode B (y,,, d) under the following condition.

(V /V cos 0= 1 (6) Namely, if the above condition is fulfiled, electrons accelerated by the voltage V and injected through the aperture S at the angle 0 are brought into contact with the electrode B on their way to the point E where the electrons would strike the electrode A. Upon the contact of the electrons with the electrode B, secondary electrons proportional in number to the impinging electrons are emitted from the electrode B at the point D in the direction perpendicular to the surface of the electrode. The secondary electrons are accelerated toward the electrode A by the electric field existing between the electrode to reach the electrode A at the point F which is opposite to the point D and apart by a distance x,,, from the aperture S. It will be understood from the formula (5) that the distance x is one half of the distance between the point B and the aperture S.

On the other hand, electrons which have an initial velocity represented by an acceleration voltage V lower than the value that satisfies the condition of the formula (6), will not be brought into contact with the electrode B, but proceed along a parabolic curve determined by the equation (2) to eventually hit the electrode A at a point G. Therefore, by providing apertures in the electrode A at the points F and G and setting electron detectors outside of the respective apertures, a group consisting of electrons having two different ranges of energy can be separately detected with an ample space between the apertures. Thus, the abovementioned difficulty involved in the conventional analyzer is eliminated, and easy construction as well as simple adjustment of a parallel plane type energy analyzer are realized.

Referring to FIG. 4 which schematically shows the structure of an embodiment of the energy analyzer of this invention, a first and a second planar electrodes 1 1 and 12 are disposed parallel to each other with a space d therebetween. The first electrode 11 is maintained at ground potential, while a negative voltage V., is applied to the second electrode 12. An aperture 13 is provided in the first electrode 11 for admitting the electrons to be analyzed. Further apertures 14 and 15 are provided also in the first electrode as outlets respectively for the secondary electrons knocked out by the electrons of larger energy and for the electrons of reduced energy. Collector electrodes 16, 17 of electron detectors (not shown) are set respectively at the front of the apertures l4, 15. For measurements in which comparatively small number of electrons should be detected, detectors of high sensitivity such as a combination of a scintillator and a photo-electronic multiplier must be used.

In the above arrangement, an electron stream which has passed through a sample membrane and therefore includes, in part, retarded or enervated electrons, is led into the space between the electrodes through the aperture 13 at an incident angle of 6. Of the total electrons, those which have not retarded by the sample membrane proceed along a path 18 and come into contact with the second electrode 12 at a point P to which the aperture 16 is opposite. Secondary electrons proportional in number to the electrons impinging on the second electrode are emitted from the second electrode at the point P in the direction perpendicular to the surface of the electrode and proceed toward the first electrode 11 to be received by the collector 16 passing through the aperture 14. The impinging primary electrons are absorbed by the electrode 12. On the other hand, electrons which have suffered an energy loss through the sample membrane proceed along another path 19 to return to the first electrode 11 and are received by the collector 17 passing through the aperture 15. Outputs of the collectors l6, 17 are led respectively to associated detector circuits (not shown) and outputs of the respective detector circuits are compared. Thus can be determined the ratio in numbers of the enervated electrons to the uninfluenced ones as reflected in the number of the secondary electrons being approximately two times as long as the space d between the electrodes, where the incident angle 0 is 45. Therefore, the only matter to be considered in making the apertures is general positioning of the apertures. As the size of the apertures is allowed to be much larger than the diameter of the electron stream, any meticulous cautions in making the apertures are not necessary for ensuring the accuracy of the analyzer.

Though the above embodiment of the invention has been described in connection with analysis of the energy of electrons, it will be clear that this invention can be also applied to the analysis of other electrified particles.

I claim:

1. An energy analyzer of parallel plane type used for analyzing the energy composition of a group of electrified particles; comprising a first planar electrode, a second planar electrode disposed parallel to said first planar electrode with a space therebetween, and means for applying a voltage between said first and second electrodes in such a manner that said second electrode is of the same polarity as that of said electrified particles in relation to said first electrode; said first electrode having a first aperture which allows said group of electrified particles to enter the space between said first and second electrodes, a second aperture positioned so as to let pass secondary electrons emitted from said second electrode at the spot designed to be bombarded by a partial group of particles having a larger energy of said group of particles and a third aperture positioned so as 0 le pass another partial group of particles having a smaller energy which proceed along a curved path through said space between said electrodes. 

1. An energy analyzer of parallel plane type used for analyzing the energy composition of a group of electrified particles; comprising a first planar electrode, a second planar electrode disposed parallel to said first planar electrode with a space therebetween, and means for applying a voltage between said first and second electrodes in such a manner that said second electrode is of the same polarity as that of said electrified particles in relation to said first electrode; said first electrode having a first aperture which allows said group of electrified particles to enter the space between said first and second electrodes, a second aperture positioned so as to let pass secondary electrons emitted from said second electrode at the spot designed to be bombarded by a partial group of particles having a larger energy of said group of particles and a third aperture positioned so as to let pass another partial group of particles having a smaller energy which proceed along a curved path through said space between said electrodes. 